1. Field of the Invention
The present invention relates to a power semiconductor device in which measures for power loss is taken.
2. Related Art
With regard to power-rectification diodes used in a power conversion system such as an inverter, it is desirable to reduce a reverse recovery loss, a surge voltage and noise at reverse-recovery operation time switched from a forward bias to a reverse bias. In terms of the reverse recovery loss among the reverse recovery loss, the surge voltage and noise, there has been proposed a technique (refer to Japanese Patent Laid-Open Pub. Nos. 9-264570, 11-26779, and 11-40822) in which a local lifetime control region is formed within the base region to shorten the reverse recovery time.
However, the present inventor has found out that the foregoing conventional techniques include the following problem. In a local lifetime control region, a hole trap level or a deep donor level is formed as a parasitic characteristic. Therefore, when a forward bias is applied to the diode, holes are trapped in the level of this kind. Upon the reverse-recovery operation time when the diode switches from the forward bias to the reverse bias, the gradient of electric-field strength in the diode is enlarged through positive charges of the trapped holes, whereby the avalanche voltage is lowered. Therefore, a voltage between a cathode and an anode is clamped at a voltage lower than a theoretical value, and occurrence of an avalanche current increases an anode current; as a result, the reverse-recovery loss increases.
Power switching devices (such as an IGBT) have the same problem. When the device changes from the ON-state to the OFF-state, an avalanche current may flow, thereby causing a considerable power loss.